The median ocellus of the giant barnacle comprises four extraordinarily large and visible photoreceptors. We can record intracellularly from the somata, axons, and presynaptic terminal regions of these decrementally-conducting receptors, as well as from postsynaptic cells in the supraesophageal ganglion. The normally-passive terminals of the receptor can be induced to give a Ca-dependent action potential in low concentrations of agents such as tetraethylammonium ion (TEA) that block K conductance. Presumably these Ca channels mediate transmitter release from the receptor. Normally the Ca current is prevented from becoming regenerative both by the TEA-sensitive K conductance and by an additional TEA-insensitive one. Studies of separately perfused or isolated somata, axons, or terminals have shown that the Ca channels and both K channels are present throughout the receptor membrane but appear to be more concentrated in the terminals. The participation of these mechanisms in signal transmission along, and transmitter release from, this nonspiking neuron is being investigated. BIBLIOGRAPHIC REFERENCE: Ross, W.N. and Stuart, A.E. Regenerative activity in the presynaptic terminal region of the barnacle photoreceptor. Biol. Bull. 151:427-428, 1976.